Display medium and display device

ABSTRACT

A display medium includes a pair of substrates, a dispersion medium sealed between the pair of substrates, a migrating particle group dispersed in the dispersion medium, and a surface layer provided on at least one of the facing surfaces of the pair of substrates and including a polymer compound that is a copolymer containing the following constitutional unit (A) and constitutional unit (B): 
     
       
         
         
             
             
         
       
     
     In the constitutional units (A) and (B), X represents a group containing a silicone chain, Ra 1  and Ra 2  each independently represent a hydrogen atom or a methyl group, Rb 2  represents an organic group containing a substituted or unsubstituted phenyl group, n1 and n2 each represent mold of the constitutional unit relative to the whole copolymer and satisfy 0&lt;n1&lt;50 and 0&lt;n2&lt;80, respectively, and n represents a natural number of 1 or more and 3 or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2010-212751 filed Sep. 22, 2010.

BACKGROUND (i) Technical Field

The present invention relates to a display medium and a display device.

SUMMARY

According to an aspect of the invention, there is provided a displaymedium including a pair of substrates at least one of which hastransparency and surfaces of which face and are disposed with a spacetherebetween, a dispersion medium sealed between the pair of substrates,a group of migrating particles dispersed in the dispersion medium so asto migrate in the dispersion medium according to an electric fieldformed between the pair of substrates, and a surface layer provided onat least one of the facing surfaces of the pair of substrates andincluding a polymer compound that is a copolymer containing thefollowing constitutional unit (A) and constitutional unit (B):

In the constitutional units (A) and (B), X represents a group containinga silicone chain, Ra₁ and Ra₂ each independently represent a hydrogenatom or a methyl group, Rb₂ represents an organic group containing asubstituted or unsubstituted phenyl group, n1 and n2 each represent mol% of the constitutional unit relative to the whole copolymer and satisfy0<n1<50 and 0<n2<80, respectively, and n represents a natural number of1 or more and 3 or less.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a schematic configuration diagram of a display deviceaccording to an exemplary embodiment; and

FIGS. 2A and 2B are explanatory views each schematically showing amigration state of a migrating particle group when a voltage is appliedbetween substrates of a display medium in a display device according toan exemplary embodiment.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are described withreference to the drawings. Members having the same operation andfunction are denoted by the same reference numeral in all drawings, andduplicate description may be omitted.

FIG. 1 is a schematic configuration diagram of a display deviceaccording to an exemplary embodiment. FIGS. 2A and 2B are explanatoryviews each schematically showing a migration state of a migratingparticle group when a voltage is applied between substrates of a displaymedium in a display device according to an exemplary embodiment.

As shown in FIG. 1, a display device 10 according to an exemplaryembodiment includes a display medium 12, a voltage applying section 16that applies a voltage to the display medium 12, and a control section18.

The display medium 12 includes a display substrate 20 serving as animage display plane, a back substrate 22 facing the display substrate 20with a space therebetween, a space member 24 that holds a specifiedspace between the substrates and that partitions the space between thedisplay substrate 20 and the back substrate 22 into plural cells, and areflecting particle group 36 having a different optical reflectioncharacteristic from that of a migrating particle group 34 sealed in eachof the cells.

Each of the cells refers to a region surrounded by the display substrate20, the back substrate 22 and the space member 24. The migratingparticle group 34 and a dispersion medium 50 are sealed in each of thecells. The migrating particle group 34 includes plural migratingparticles and is dispersed in the dispersion medium 50 so as to movethrough the gaps between reflection particles of the reflecting particlegroup 36 between the display substrate 20 and the back substrate 22according to the intensity of an electric field formed in each of thecells.

As the migrating particle group 34, a migrating particle group 34A and amigrating particle group 34B that assumes a different color and has adifferent charge polarity from the migrating particle group 34A areused.

The space member 24 may be provided to correspond to each pixel when animage is displayed on the display medium 12, and the cells may be formedto correspond to respective pixels so that the display medium 12 isconfigured to display for each of the pixels.

For the sake of simple description, the exemplary embodiment isdescribed with reference to the drawings for one cell. Hereinafter, eachcomponent is described in detail.

First, the pair of substrates is described.

The display substrate 20 includes a surface electrode 40 and a surfacelayer 42 laminated in order on a support substrate 38. The backsubstrate 22 includes a back electrode 46 and a surface layer 48laminated in order on a support substrate 44.

The display substrate 20 or both the display substrate 20 and the backsubstrate 22 have transparency. Here, in the exemplary embodiment,“transparency” represents that a visible light transmittance is 60% ormore.

The support substrate is described.

Examples of materials for the support substrate 38 and the supportsubstrate 44 include glass and plastics such as polyethyleneterephthalate resins, polycarbonate resins, acryl resins, polyimideresins, polyester resins, epoxy resins, polyether sulphone resins, andthe like.

The electrodes are described.

Examples of materials for the surface electrode 40 and the backelectrode 46 include oxides of indium, tin, cadmium, antimony, and thelike; compound oxides such as ITO and the like; metals such as gold,silver, copper, nickel, and the like; and organic materials such aspolypyrrole, polythiophene, and the like. The surface electrode 40 andthe back electrode 46 may be made of any one of a single-layer film, amixed film, and a composite film of these materials. The thickness ofthe surface electrode 40 and the back electrode 46 is preferably, forexample, 100 Å or more and 2000 Å or less. The back electrode 46 and thesurface electrode 40 may be formed in, for example, a matrix shape or astripe shape.

In addition, the surface electrode 40 may be buried in the supportsubstrate 38, and the back electrode 46 may be buried in the supportsubstrate 44. In this case, the material of the support substrate 38 andthe support substrate 44 is selected according to the composition ofeach of the migrating particles in the migrating particle group 34, andthe like.

The back electrode 46 and the surface electrode 40 may be disposed toseparate from the display substrate 20 and the back substrate 22,respectively, outside the display medium 12.

Although, in the above description, the electrodes (the surfaceelectrode 40 and the back electrode 46) are provided on both the displaysubstrate 20 and the back substrate 22, an electrode may be provided onany one of the substrates so as to perform active matrix drive.

In addition, TFT (thin-film transistor) may be provided for each pixelof the support substrate 38 and the support substrate 44 in order toperform active matrix drive. TFT is preferably provided on the backsubstrate 22, not the display substrate 20.

The surface layers are described.

The surface layer 42 and the surface layer 48 are provided on the facingsurfaces of the display substrate 20 and the back substrate 22. Also, asurface layer 25 is provided on the surface (inner surface of each cell)of the space member 24.

In this exemplary embodiment, description is made of the case in whichthe surface layers (the surface layer 42 and the surface layer 48) areprovided on the facing surfaces of the display substrate 20 and the backsubstrate 22. However, a surface layer may be provided on only one ofthe facing surfaces of the display substrate 20 and the back substrate22. From the viewpoint of control of the migration start voltage ofmigrating particles adhering to the substrates, it is desirable toprovide the surface layers (the surface layers 42 and 48) on both theopposing surfaces of the display substrate 20 and the back substrate 22.

In addition, from the viewpoint of suppression of image defects due toadhesion of migrating particles, it is desirable to provide the surfacelayer 42 on the opposing surface on at least the display substrate 20side. In addition, when the surface layer 25 is provided on the surface(inner surface of each cell) of the space member 24, adhesion of themigrating particles to the space member 24 is suppressed as comparedwith the case in which the surface layer 25 is not provided on the spacemember 24. As a result, an increase in migrating particles which do notcontribute to display is suppressed. Namely, it is desirable to providethe surface layer 42 on the opposing surface on at least the displaysubstrate 20 side and most desirable to provide the surface layers onall the pair of substrates and the space member (i.e., the inner wallsof the cells surrounded by the substrates and the space member).

Each of the surface layers 42, 48, and 25 is composed of a polymercompound having a silicone chain. Specifically, for example, each of thesurface layers 42, 48, and 25 is formed by forming the polymer compoundhaving a silicone chain on each of the facing surfaces by treatment ofchemically bonding the polymer compound having a silicone chain to eachof the facing surfaces of the display substrate 20 and the backsubstrate 22 and the facing surfaces of the space member 24, ortreatment of coating each of the facing surfaces with the polymercompound having a silicone chain (i.e., forming a film of the polymercompound).

As the polymer compound having a silicone chain, a copolymer containingthe following constitutional unit (A) and constitutional unit (B) isused.

In the constitutional units (A), X represents a group containing asilicone chain, Ra₁ and Ra₂ each independently represent a hydrogen atomor a methyl group, Rb₂ represents an organic group containing asubstituted or unsubstituted phenyl group, n1 and n2 each represent mol% of the constitutional unit relative to the whole copolymer and satisfy0<n1<50 and 0<n2<80, respectively, and n represents a natural number of1 or more and 3 or less.

A group containing a silicone chain and represented by X in theconstitutional units (A) and (B) is, for example, a group containing astraight or branched silicone chain (siloxane chain in which two or moreSi—O bonds are linked), preferably a group containing a dimethylsiloxanechain in which two or more dimethylsiloxane structures (—Si(CH₃)₂—O—)are linked and a portion (a portion of —CH₃) may be substituted by asubstituent.

Specific examples of a group containing a silicone chain and representedby X include groups represented by the following structural formulae(X1) and (X2):

In the structural formulae (X1) and (X2), R₁ represents a hydroxylgroup, a hydrogen atom, or an alley group having 1 to 10 carbon atoms,and n represents an integer of 1 to 1000.

In the structural formulae (X1) and (X2), an alkyl group represented byR₁ is an alkyl group having 1 to 10 carbon atoms, preferably an alkylgroup having 1 to 4 carbon atoms, and more preferably a methyl group.

n is an integer of 1 to 1000, preferably 2 to 200, and more preferably 5to 100.

Examples of an organic group having a substituted or unsubstitutedphenyl group and represented by Rb₂ in the constitutional unit (B)include an organic group having a substitute or unsubstituted phenylgroup at an end thereof (an end not bonded to O in the constitutionalunit (B)).

Specific examples of an organic group having a substituted orunsubstituted phenyl group and represented by Rb₂ include —Rb₂₁-O-Ph(wherein Rb₂₁ represents an alkylene group having 1 or more and 11 orless carbon atoms or an alkylene group substituted by a hydroxyl groupand having 1 or more and 11 or less carbon atoms, and Ph represents asubstituted or unsubstituted phenyl group).

An alkylene group represented by Rb₂₁ is an alkylene group having 1 ormore and 11 or less carbon atoms and preferably an alkylene group having1 or more and 4 or less carbon atoms.

The alkylene group may be substituted by one hydroxyl group or two ormore hydroxyl groups.

Examples of a substituent introduced to a phenyl group include an alkylgroup having 1 or more and 4 or less carbon atoms, a hydroxyl group, andthe like.

The alkyl group represented by each of the characters may be a straightchain or branched. Specific examples thereof include a methyl group, anethyl group, an isopropyl group, a butyl group, an isopentyl group, anamyl group, a hexyl group, a cyclohexyl group, an octyl group, anethylhexyl group, an isononyl group, a decyl group, and the like. Thealkyl group is selected according to a target carbon number.

The alkylene group represented by each of the characters may be astraight chain or branched. Specific examples thereof include amethylene group, an ethylene group, an isopropylene group, a butylenegroup, an isopentylene group, an amylene group, a hexylene group, acyclohexylene group, an octylene group, an ethylhexylene group, anisononylene group, a decylene group, and the like.

The alkylene group is selected according to a target carbon number.

In the constitutional units (A) and (B), n1 and n2 have the relations of0<n1<50 and 0<n2<80, preferably the relations of 0<n1<20 and 0<n2<60,and more preferably the relations of 0<n1<10 and 0<n2<50, respectively.

In this case, when n1 and n2 have the relation of n1<n2, the migrationstart voltage of migrating particles adhering to the substrates may beincreased.

On the other hand, when n1 and n2 have the relation of n1>n2, themigration start voltage of migrating particles adhering to thesubstrates may be decreased.

Namely, control of the migration start voltage of migrating particlesadhering to the substrate is realized by controlling the component ratio(molar ratio) between the constitutional unit (A) and the constitutionalunit (B).

In the constitutional unit (A), n represents a natural number of 1 to 3.

Specific examples of a monomer constituting the constitutional unit (A)in the polymer compound having a silicone chain include adimethylsilicone monomer having a (meth)acrylate group at one end (forexample, Silaplane: FM-0711, FM-0721, and FM-0725 manufactured by ChissoCorporation, and X-22-174DX, X-22-2426, and X-22-2475 manufactured byShinetsu Silicone Co., Ltd.). Among these, Silaplane: FM-0711, FM-0721,and FM-0725 are desired.

Specific examples of a monomer constituting the constitutional unit (B)include 2-phenoxyethyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate, andthe like.

The polymer compound having a silicone chain may be a copolymercontaining another constitutional unit (e) together with theconstitutional units (A) and (B). When the polymer compound contains theother constitutional unit (C) as a constitutional unit of a copolymer,the film forming properties of the surface layer are secured, and filmdeterioration by the dispersion medium is suppressed.

Examples of a monomer constituting the other constitutional unit (C)include nonionic monomers. That is, the other constitutional unit (C) isa constitutional unit derived from a nonionic monomer.

Examples of the nonionic monomer include (meth)acrylonitrile,(meth)acrylic acid alkyl esters, (meth)acrylamide, ethylene, propylene,butadiene, isoprene, isobutylene, N-dialkyl-substituted(meth)acrylamides, styrene, vinylcarbazole, styrene derivatives,polyethylene glycol mono (meth)acrylate, vinyl chloride, vinylidenechloride, isoprene, butadiene, vinyl pyrrolidone, hydroxyethyl(meth)acrylate, hydroxybutyl (meth)acrylate, and the like.

In the description, the expression “(meth)acrylate” or the like includes“acrylate” and “methacrylate”.

When the polymer compound having a silicone chain is a copolymercontaining the other constitutional unit (C) together with theconstitutional units (A) and (B), mol % (denoted by n3) of the otherconstitutional unit (C) based on the whole copolymer is, for example,0<n3<99 and preferably 10<n3<97.

In the polymer compound having a silicone chain, ratio of theconstitutional unit (A) to the constitutional unit (B) (mass ratio:constitutional unit (A)/constitutional unit (B)) is, for example, 1/99or more and 99/1 or less, preferably 5/95 or more and 95/5 or less, andmore preferably 10/90 or more and 90/10 or less.

When the polymer compound having a silicone chain contains the otherconstitutional unit (C), the polymerization ratio (mass ratio:(constitutional unit (A)+constitutional unit (B))/other constitutionalunit (C)) of the total of the constitutional unit (A) and theconstitutional unit (B) to the other constitutional unit (C) is, forexample, 10/90 or more and 90/10 or less, preferably 15/85 or more and85/15 or less, and more preferably 20/80 or more and 80/20 or less.

Specific examples of the polymer compound having a silicone chaininclude compounds given below but are not limited to these compounds.

The specific examples given below are examples of the polymer compoundfurther containing the other constitutional unit (C).

Examples of a terminal group of the polymer compound having a siliconechain include a hydroxyl group, a methyl group, an alkyl group, acarboxyl group, and the like.

The weight-average molecular weight of the polymer compound having asilicone chain is preferably 100 or more and 1,000,000 or less, morepreferably 400 or more and 1,000,000 or less.

The weight-average molecular weight is measured by a static lightscattering method or size-exclusion column chromatography, and a valuedescribed in the specification is measured by this method.

The thickness of the surface layers (the surface layers 42, 48, and 25)composed of the polymer compound having a silicone chain is, forexample, 0.001 μm or more and 10 μm or less, and preferably 0.01 μm ormore and 1 μm or less.

The space member is described.

The space member 24 provided for maintaining the space between thedisplay substrate 20 and the back substrate 22 is composed of, forexample, a thermoplastic resin, a thermosetting resin, an electron raycurable resin, a photocurable resin, rubber, a metal, or the like.

The space member 24 may be integrated with any one of the displaysubstrate 20 and the back substrate 22. In this case, the space member24 is formed by etching or laser-beam machining of the support substrate38 or 44 or pressing or printing of the support substrate 38 or 44 usinga previously formed pattern.

In this case, the space member 24 is formed on one or both of thedisplay substrate 20 side and the back substrate 22 side.

The space member 24 may be colored or colorless but is colorless andtransparent. In this case, the space member 24 is composed of atransparent resin such as polystyrene, polyester, acryl, or the like.

The granular space member 24 is also transparent, and particles of atransparent resin such as polystyrene, polyester, acryl, or the like, orglass particles are used.

The term “transparent” represents having a visible light transmittanceof 600 or more.

The dispersion medium is described.

The dispersion medium 50 in which the migrating particle group 34 isdispersed is an insulating liquid. Here, “insulating” represents avolume resistivity of 10¹¹ Ωcm or more. This applies to descriptionbelow.

Specific examples of the insulating liquid include hexane, cyclohexane,toluene, xylene, decane, hexadecane, kerosene, paraffin, isoparaffin,silicone oil, high-purity petroleum, ethylene glycol, alcohols, ethers,esters, dimethylformamide, dimethylacetamide, dimethyl sulfoxide,1-methyl-2-pyrrolidone, N-methylformamide, acetonitrile,tetrahydrofuran, propylene carbonate, ethylene carbonate, benzine,diisopropylnaphthalene, olive oil, trichlorotrifluoroethane,tetrachloroethane, dibromotetrafluoroethane, and the like; and mixturesthereof. Among these, silicone oil is desirably used. By using siliconeoil, the silicone chain of the polymer compound constituting the surfacelayers is liable to be present on the dispersion medium 50 side, therebyimproving controllability of a migration start voltage of migratingparticles adhering to the substrates and the effect of suppressingparticle adhesion.

In addition, water (so-called pure water) from which imurities areremoved to achieve a volme resistivity described below is also used asthe dispersion medium 50. The volume resistivity is preferably 10³ Ωcmor more, more preferably 10⁷ Ωcm or more and 10¹⁹ Ωcm or less, and stillmore preferably 10¹⁰ Ωcm or more and 10¹⁹ Ωcm or less. With a volumeresistivity within this range, an electric field is more effectivelyapplied to the migrating particle group 34, and the occurrence ofbubbles due to electrolysis of a liquid by electrode reaction issuppressed, thereby decreasing deterioration in migrationcharacteristics of the migrating particles at each time of currentapplication and imparting excellent repetition stability.

According to demand, an acid, an alkali, a salt, a dispersionstabilizer, a stabilizer for anti-oxidation and ultraviolet absorption,an antibacterial agent, a preservative agent, and the like may be addedto the insulating liquid, but such agents are added so that the volumeresistivity is within the above specified range.

Further, an anionic surfactant, a cationic surfactant, an amphotericsurfactant, a nonionic surfactant, a fluorine-based surfactant, asilicone surfactant, metal soap, an alkyl phosphate, succinic acidimide, or the like may be added to the insulating liquid.

Specific examples of ionic and nonionic surfactants include thefollowing: Examples of the nonionic surfactant include polyoxyethylenenonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylenedodecylphenyl ether, polyoxyethylene alkyl ethers, polyoxyethylene fattyacid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fattyacid esters, fatty acid alkylolamides, and the like. Examples of theanionic surfactant include alkylbenzenesulfonic acid salts,alkylphenylsulfonic acid salts, alkylnaphthalenesulfonic acid salts,higher fatty acid salts, sulfuric acid ester salts of higher fatty acidesters, sulfonic acid ester salts of higher fatty acid esters, and thelike. Examples of the cationic surfactant include primary or tertiaryamine salts, quaternary ammonium salts, and the like. The amount of sucha charge control agent is preferably 0.01% by mass or more and 20% bymass or less, particularly preferably 0.05% by mass or more and 10% bymass or less, based on the particle solid content. When the amount isless than 0.01% by mass, the desired charge control effect isinsufficient, while when the amount exceeds 20% by mass, theconductivity of a developer may be excessively increased, therebycausing difficulty in use.

As the dispersion medium 50, a polymeric resin may be combined with theinsulating liquid. As the polymeric resin, a high-molecular gel, ahigh-molecular polymer, or the like may be used.

Examples of the polymeric resin include high-molecular gels derived fromnatural polymers, such as agarose, agaropectin, amylose, sodiumalginate, propylene glycol alginate, isolichenan, insulin, ethylcellulose, ethylhydroxyethyl cellulose, curdlan, casein, carrageenan,carboxymethyl cellulose, carboxymethyl starch, callose, agar, chitin,chitosan, silk fibroin, guar gum, quince seed, crown gall polysaccaride,glycogen, glucomannan, keratan sulfate, keratin protein, collagen,cellulose acetate, gellan gum, sizofuran, gelatin, ivory palm mannan,tunicin, dextran, dermatan sulfate, starch, tragacanth gum, nigeran,hyaluronic acid, hydroxyethyl cellulose, hydroxypropyl cellulose,pustulan, funoran, decomposed xyloglucan, pectin, porphyran, methylcellulose, methyl starch, laminaran, lichenan, lentinan, locust beangum, and the like; and almost all high-molecular gels of syntheticpolymers.

Further, a polymer containing, in its repeat unit, a functional groupsuch as an alcohol, a ketone, an ether, an ester, or an amide may beused. Examples of such a polymer include polyvinyl alcohol,poly(meth)acrylamide and derivatives thereof, polyvinyl pyrrolidone,polyethylene oxide, and copolymers of these polymers.

Among these, from the viewpoint of production stability, electrophoreticproperties, and the like, gelatin, polyvinyl alcohol,poly(meth)acrylamide, or the like is preferably used.

In addition, when a coloring agent below is mixed with the dispersionmedium 50, a color different from that of the migrating particle group34 is displayed on the display medium 12. For example, when the color ofthe migrating particle group 34 is black, white and black colors aredisplayed on the display medium 12 by mixing the coloring agent thatexhibits while color.

Examples of the coloring agent mixed with the dispersion medium 50include known coloring agents such as carbon black, titanium oxide,magnesium oxide, zinc oxide, phthalocyanine copper-based cyan colorants,azo-based yellow colorants, azo-based magenta colorants,quinacridone-based magenta colorants, red colorants, green colorants,blue colorants, and the like. Specifically, typical examples thereofinclude aniline blue, charcoal blue, chrome yellow, ultramarine blue,Dupont oil red, quinoline yellow, methylene blue chloride,phthalocyanine blue, malachite green oxalate, lamp black, rose bengal,C. I. Pigment Red 48:1, C. I. Pigment Red 122, C. I. Pigment Red 57:1,C.I. Pigment Yellow 97, C. I. Pigment Blue 15:1, C. I. Pigment Blue15:3, and the like.

The migrating particle group 34 moves in the dispersion medium 50. Ifthe viscosity of the dispersion medium 50 is excessively high, the forceacting on the back substrate 22 and the display substrate 20 greatlyvaries, thereby failing to determine a threshold of movement of theparticles (migration of the particles) according to an electric field.Therefore, the viscosity of the dispersion medium 50 may be controlled.

The viscosity of the dispersion medium 50 in an environment at atemperature of 20° C. may be 0.1 mPa·s or more and 100 mPa·s or less,preferably 0.1 mPa·s or more and 50 mPa·s or less, and more preferably0.1 mPa·s or more and 20 mPa·s or less, from the viewpoint of the movingspeed (migration speed) of the migrating particles, i.e., the displayrate.

By adjusting the viscosity of the dispersion medium 50 to be in therange of 0.1 mPa·s or more and 100 mPa·s or less, variations in theadhesive force between the migrating particle group 34 dispersed in thedispersion medium 50 and the display substrate 20 or the back substrate22, the flow resistance, and the electrophoresis time are suppressed.

The viscosity of the dispersion medium 50 is adjusted by, for example,regulating the molecular weight, structure, composition, and the like ofthe dispersion medium. The viscosity is measured with a B-8L viscometermanufactured by Tokyo Keiki Inc.

Next, the migrating particle group is described. The migrating particlegroup 34 includes plural migrating particles, and each of the migratingparticles is charged positively or negatively. When a predeterminedvoltage is applied between the surface electrode 40 and the backelectrode 46 (i.e., between the display substrate 20 and the backsubstrate 22), an electric field of a predetermined strength or more isformed between the display substrate 20 and the back substrate 22, andconsequently the migrating particles move in the dispersion medium 50.

A change in display color in the display medium 12 is caused by themovement of migrating particles constituting the migrating particlegroup 34.

Examples of the migrating particles of the migrating particle group 34include glass beads, insulating metal oxide particles of alumina,titanium oxide, and the like, thermoplastic or thermosetting resinparticles, the resin particles containing a coloring agent fixed to thesurfaces thereof, particles of thermoplastic or thermosetting resincontaining a coloring agent, metal colloid particles having a plasmoncoloring function, and the like.

Examples of a thermoplastic resin used for producing the migratingparticles include homopolymers or copolymers of styrenes such asstyrene, chlorostyrene, and the like; monoolefins such as ethylene,propylene, butylene, isoprene, and the like; vinyl esters such as vinylacetate, vinyl propionate, vinyl benzoate, vinyl butyrate, and the like;α-methylene aliphatic monocarboxylic acid esters such as methylacrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octylacrylate, phenyl acrylate, methyl methacrylate, ethyl methacrylate,butyl methacrylate, dodecyl methacrylate, and the like; vinyl etherssuch as vinyl methyl ether, vinyl ethyl ether, vinyl butyl ether, andthe like; vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone,vinyl isopropenyl ketone, and the like.

Examples of a thermosetting resin used for producing the migratingparticles include cross-linked resins such as cross-linked copolymerscomposed of divinyl benzene as a main component, cross-linked polymethylmethacrylate, and the like, phenol resins, urea resins, melamine resins,polyester resins, silicone resins, and the like. Particularly typicalbinder resins include polystyrene, styrene-alkyl acrylate copolymers,styrene-alkyl methacrylate copolymers, styrene-acrylonitrile copolymers,styrene-butadiene copolymers, styrene-maleic anhydride copolymers,polyethylene, polypropylene, polyesters, polyurethane, epoxy resins,silicone resins, polyamides, modified rosin, paraffin wax, and the like.

As the coloring agent, an organic or inorganic pigment, an oil-solubledye, or the like may be used. Examples thereof include known coloringagents such as magnetic powders of magnetite, ferrite, and the like,carbon black, titanium oxide, magnesium oxide, zinc oxide, copperphthalocyanine-based cyan coloring agents, azo yellow coloring agents,azo magenta coloring agents, quinacridone magenta coloring agents, redcoloring agents, green coloring agents, blue coloring agents, and thelike. Specifically, typical examples thereof include aniline blue,charcoal blue, chrome yellow, ultramarine blue, Dupont oil red,quinoline yellow, methylene blue chloride, phthalocyanine blue,malachite green oxalate, lamp black, rose bengal, C.I. Pigment Red 48:1,C. I. Pigment Red 122, C. I. Pigment Red 57:1, C. I. Pigment Yellow 97,C. I. Pigment Blue 15:1, C. I. Pigment Blue 15:3, and the like.

The resin of the migrating particles may be mixed with a charge controlagent according to demand. Examples s charge control agent include knowncharge control agents used for electrophotographic toner materials, suchas cetylpyridyl chloride, quaternary ammonium salts such as BONTRONP-51, BONTRON P-53, BONTRON E-84, and BONTRON E-81 (manufactured byOrient Chemical Industries Co., Ltd.), and the like, salicylic acidmetal complexes, phenol condensates, tetraphenyl compounds, metal oxideparticles, metal oxide particles that are surface-treated with variouscoupling agents, and the like.

A magnetic material may be mixed in the migrating particles or thesurfaces thereof. As the magnetic material, a color-coated inorganicmagnetic material or organic magnetic material is used according todemand. A transparent magnetic material, particularly a transparentorganic magnetic material, may be used because it does not inhibit thecolor development of a coloring pigment and has a smaller specificgravity than that of an inorganic magnetic material.

As a colored magnetic powder, a colored magnetic powder having a smallparticle diameter described in, for example, Japanese Unexamined PatentApplication Publication No. 2003-131420 may be used. A colored magneticpowder including magnetic particles as nuclei and colored layerslaminated on the surfaces of the magnetic particles is used. A coloredlayer formed by opaquely coloring a magnetic powder with a pigment orthe like may be selected, but, for example, a light interference thinfilm may be used. The light interference thin film is a thin filmcomposed of an achromatic material, such as SiO₂, TiO₂, or the like, andhaving a thickness equal to the wavelength of light, so that light iswavelength-selectively reflected by light interference within the thinfilm.

An external additive may be adhered to the surfaces of the migratingparticles. The external additive is transparent so as not to influencethe color of the migrating particles.

As the external additive, inorganic particles of a metal oxide such assilicon oxide (silica), titanium oxide, alumina, or the like are used.In order to control the chargeability, mobility, and environmentdependence of the particles, the particles may be surface-treated with acoupling agent or silicone oil.

Examples of the coupling agent include positively chargeable couplingagents such as aminosilane coupling agents, aminotitanium couplingagents, nitrile coupling agents, and the like; negatively chargeablecoupling agents such as silane coupling agents, titanium couplingagents, epoxysilane coupling agents, acrylsilane coupling agents, andthe like, which do not contain nitrogen atoms (composed of atoms otherthan nitrogen). Examples of the silicone oil include positivelychargeable silicone oil such as amino-modified silicone oil and thelike; and negatively chargeable silicone oil such as dimethyl siliconeoil, alkyl-modified silicone oil, α-methylsulfone-modified silicone oil,methylphenyl silicone oil, chlorophenyl silicone oil, fluorine-modifiedsilicone coil and the like. The external additive is selected accordingto desired resistance.

Among the above-described external additives, well known hydrophobicsilica and hydrophobic titanium oxide are desired, and particularly atitanium compound produced by reaction of TiO(OH)₂ and a silane compoundsuch as a silane coupling agent as described in Japanese UnexaminedPatent Application Publication No. 10-3177 is desired. As the silanecompound, any type of chlorosilane, alkoxysilane, silazane, and aspecial silylating agent may be used. The titanium compound is producedby reacting TiO(OH)₂ formed in a wet process with the silane compound orsilicone oil and drying the product. Since the titanium compound is notsubjected to a firing process at several hundred degrees, Ti strongbonds are not formed and the migrating particles are in a primaryparticle state without aggregation. Further, since TiO(OH)₂ is reacteddirectly with the silane compound or silicone oil, the amount oftreatment with the silane compound or silicone oil may be increased, andthus charging properties may be controlled by adjusting the amount oftreatment with the silane compound, and the chargeability imparted ismore improved than usual titanium oxide.

The primary particles of the external additive generally have a size of1 nm or more and 100 nm or less, preferably 5 nm or more and 50 nm orless, but the size is not limited to this.

The mixing ratio of the external additive to the migrating particles isadjusted in view of balance between the particle diameter of themigrating particles and the particle diameter of the external additive.When the amount of the external additive added is excessively large, atleast a portion of the external additive is separated from the surfacesof the migrating particles and adheres to the surfaces of the othermigrating particles, failing to achieve desired charge characteristics.The amount of the external additive is generally 0.01 part by mass ormore and 3 parts by mass or less and preferably 0.05 part by mass ormore and 1 part by mass or less based on 100 parts by mass of themigrating particles.

The external additive may be added to any one of plural types ofmigrating particles or added to plural types or all types of migratingparticles. When the external additive is added to the surfaces of allmigrating particles, the external additive is inserted into the surfacesof the migrating particles by applying impact force or the externaladditive is strongly adhered to the surfaces of the migrating particlesby heating the surfaces of the migrating particles. As a result, theexternal additive is prevented from separating from the migratingparticles, and aggregates of the external additive, which are difficultto separate with an electric field, are prevented from being formed dueto strong aggregation of the external additive with heteropolarity,thereby preventing deterioration in image quality.

As a method for forming the migrating particle group 34, any usual knownmethod may be used. An example of a usable method includes, as describedin Japanese Unexamined Patent Application Publication No. 7-325434,weighing a resin, a pigment, and a charge control agent at an intendedmixing ratio, melting the resin by heating, adding and mixing thepigment with the melted resin, dispersing the pigment in the meltedresin, cooling the mixture, preparing particles with a mill such as ajet mill, a hammer mill, a turbo mill, or the like, and dispersing theresultant particles in a dispersion medium. Alternatively, particlescontaining a charge control agent may be prepared by a polymerizationmethod such as suspension polymerization, emulsion polymerization,dispersion polymerization, or the like or a coacervation, meltdispersion, or emulsion aggregation method, and then dispersed in adispersion medium to prepare a particle dispersion medium. Further, whenthe resin has plasticity, a particle forming method uses a properapparatus for dispersing and kneading raw materials such as the resin, acoloring agent, a charge control agent, and a dispersion medium at a lowtemperature at which the dispersion medium does not boil and which islower than the decomposition point of at least one of the resin, thecharge control agent, and the coloring agent. Specifically, the pigment,the resin, and the charge control agent are melted in the dispersionmedium using a planetary mixer, a kneader, or the like, and particlesare produced by solidification/precipitation by cooling the melt mixtureunder stirring using the temperature dependence of the solventsolubility of the resin.

Another method may be used, in which the raw materials are placed in anappropriate vessel provided with a granular medium for dispersion andkneading, for example, an attritor or a vibrating mill such as a heatedball mill, and the raw materials are dispersed and kneaded in the vesselwithin a desired temperature range of, for example, 80° C. or more and160° C. or less. As the granular medium, steel such as stainless steelor carbon steel, alumina, zirconia, silica, or the like may be used. Inproducing the migrating particles by this method, the raw materialswhich are previously put into a flowing state are further dispersed inthe vessel using the granular medium, and then the dispersion medium iscooled to precipitate the resin containing the coloring agent from thedispersion medium. During cooling and after cooling, the granular mediumcontinuously generates shear and/or impact while maintaining a movingcondition, thereby further decreasing the particle size.

The content (content (% by mass) relative to the total mass in a cell)of the migrating particle group 34 is not particularly limited as longas a desired hue is obtained. For the display medium 12, it is effectiveto control the content according to the cell thickness (i.e., thedistance between the display substrate 20 and the back substrate 22).Namely, in order to obtain a desired hue, the content decreases as thecell thickness increases, and the content increases as the cellthickness decreases. The content is generally 0.01% by mass or more and50% by mass or less.

The reflecting particle group is described.

The reflecting particle group 36 include reflecting particles havingdifferent optical reflection characteristic from that of the migratingparticle group 34 and functions as a reflecting member which displays adifferent color from that of the migrating particle group 34. Inaddition, the reflecting particle group 36 also has the function as aspace member which allows movement between the display substrate 20 andthe back substrate 22 without inhibiting the movement. Namely, theparticles in the migrating particle group 34 migrate (move) from theback substrate 22 side to the display substrate 20 side or from thedisplay substrate 20 side to the back substrate 22 side through thespaces in the reflecting particle group 36. As the color of thereflecting particle group 36, for example, while or black color may beselected so as to become a background color, but another color may beselected. Further, the reflecting particle group 36 may be an unchargedparticle group (i.e., a particle group which does not move according toan electric field) or a charged particle group (i.e., a particle groupwhich moves according to an electric field). In this exemplaryembodiment, the case in which the reflecting particle group 36 is anuncharged particle group and is white in color is described, but thereflecting particle group 36 is not limited to this.

Examples of the particles of the reflecting particle group 36 includeparticles formed by dispersing a white pigment (e.g., titanium oxide,silicon oxide, zinc oxide, or the like) in a resin (e.g., polystyreneresin, polyethylene resin, polypropylene resin, polycarbonate resin,polymethyl methacrylate resin (PMMA), acryl resin, phenol resin,formaldehyde resin, or the like); particles of resins such aspolystyrene, polyethylene, polyvinylnaphthalene, and the like. Whenparticles other than white particles are used as the particles of thereflecting particle group 36, for example, the above-described resinparticles including a pigment or dye of desired color may be used. Thepigment or dye may be a general pigment or dye used for printing ink andcolor toner as long as it has RGB or YMC color.

The reflecting particle group 36 is sealed between the substrates by,for example, an ink jet method. When the reflecting particle group 36 isfixed, for example, the reflecting particle group 36 is sealed and thenheated (and pressed according to demand) to melt the surface layers ofthe reflecting particle group 36 so as to maintain the spaces betweenthe particles.

The size of the cells in the display medium 12 is closely related toresolution of the display medium 12, and the resolution of an imagedisplayed on the display medium 12 increases as the cell size decreases.The cell length is generally about 10 μm or more and 1 mm or less in theplanar direction of the display substrate 20 of the display medium 12.

To fix the display substrate 20 and the back substrate 22 with the spacemember 24 provided therebetween, a combination of bolts and nuts or afixing method such as a clamp, a clip, a substrate fixing frame, or thelike may be used. Also, a fixing method such as an adhesive, heatmelting, ultrasonic bonding, or the like may be used.

The display medium 12 configured as described above is used forimage-storing and rewriting devices, for example, a bulletin board, acircular, an electronic blackboard, an advertisement, a signboard, aflashing indicator, an electronic paper, an electronic newspaper, anelectronic book, a document sheet in common use as a copy machine andprinter, and the like.

As described above, the display device 10 according to this exemplaryembodiment includes the display medium 12, the voltage applying section16 which applies a voltage to the display medium 12, and the controlsection 18 (refer to FIG. 1).

The voltage applying section 16 is electrically connected to the surfaceelectrode 40 and the back electrode 46. In the exemplary embodiment, thecase in which both the surface electrode 40 and the back electrode 46are electrically connected to the voltage applying section 16 isdescribed. However, one of the surface electrode 40 and the backelectrode 46 may be grounded, and the other may be connected to thevoltage applying section 16.

The voltage applying section 16 is connected to the control section 18so as to give and receive signals.

The control section 18 may be configured as a micro computer includingCPU (Central Processing Unit) that controls the operation of the wholedevice, RAM (Random Access Memory) in which various data is temporarilystored, and ROM (Read Only Memory) in which various programs such as acontrol program for controlling the whole device are previously stored.

The voltage applying section 16 is a voltage applying device forapplying a voltage to the surface electrode 40 and the back electrode 46and applies a voltage between the surface electrode 40 and the backelectrode 46 according to control by the control section 18.

Next, the operation of the display device 10 is described. The operationis described according to the operation of the control section 18.

Here, description is made of the case in which in the migrating particlegroup 34 sealed in the display medium 12, the migrating particle group34A is charged to negative polarity, and the migrating particle group34B is charged to positive polarity. In addition, the dispersion medium50 is transparent, and the reflecting particle group 36 is white. Thatis, in this exemplary embodiment, description is made of the case inwhich the display medium 12 displays a color exhibited by movement ofthe migrating particle group 34A and the migrating particle group 34B,and white color is displayed as a background color.

First, an initial operation signal is output to the voltage applyingsection 16 to apply a voltage for a specified time (T1) so that thesurface electrode 40 serves as a negative electrode, and the backelectrode 46 serves as a positive electrode. When a voltage of athreshold voltage or more, at which a concentration change is finishedon the negative electrode, is applied between the substrates, themigrating particles constituting the migrating particle group 34Acharged to negative polarity move to the back substrate 22 side andreach the back substrate 22 (refer to FIG. 2A). On the other hand, themigrating particles constituting the migrating particle group 34Bcharged to positive polarity move to the display substrate 20 side andreach the display substrate 20 (refer to FIG. 2A).

In this case, on a while background that is the color of the reflectingparticle group 36, a color exhibited by the migrating particle group 34Bis visually observed as the color of the display medium 12 from thedisplay substrate 20 side. The migrating particle group 34A is shieldedby the reflecting particle group 36 and is hard to observe visually.

The time T1 as information which indicates a voltage application time inthe initial operation may be previously stored in the memory such as ROM(not shown in the drawing) in the control section 18. When processing isexecuted, the information which indicates the specified time may beread.

Next, a voltage with polarities opposite to the voltage applied betweenthe substrates is applied between the surface electrode 40 and the backelectrode 46 so that the surface electrode 40 serves as a positiveelectrode and the back electrode 46 serves as a negative electrode. As aresult, the migrating particle group 34A charged to negative polaritymove to the display substrate 20 side and reach the display substrate 20(refer to FIG. 2B). On the other hand, the migrating particlesconstituting the migrating particle group 34B charged to positivepolarity move to the back substrate 22 side and reach the back substrate22 (refer to FIG. 2B).

In this case, on a while background of the color of the reflectingparticle group 36, a color exhibited by the migrating particle group 34Ais visually observed as the color of the display medium 12 observed fromthe display substrate 20 side. The migrating particle group 34B isshielded by the reflecting particle group 36 and is hard to observevisually.

Therefore, in the display device 10 (display medium 12) according to theexemplary embodiment, the migrating particle group 34 (the migratingparticle group 34A and the migrating particle group 34B) reaches andadheres to the display substrate 20 or the back substrate 22, therebyperforming display.

In the display device 10 (display medium 12) according to theabove-described exemplary embodiment, the facing surfaces of the displaysubstrate 20 and the back substrate 22 have the surface layer 42 and thesurface layer 48, respectively, each of which includes the specifiedpolymer compound having a silicone chain (polymer compound composed of acopolymer containing the constitutional unit (A) and the constitutionalunit (B)). Therefore, the migration start voltage of the migratingparticle group 34 (migrating particles) adhering to the substrates maybe controlled.

Although the reason for this is not known, it is considered thatadhesive force of the negatively charged migrating particles isincreased by using a surface layer containing an organic group having aphenyl group.

Therefore, the migration start voltage of the migrating particle group34 (migrating particles) adhering to the substrates may be controlled.

The migration start voltage refers to a voltage (so-called thresholdvoltage) for applying electric field strength to start migration(movement) of the migrating particle group 34 (migrating particles).

In addition, when the facing surfaces of the display substrate 20 andthe back substrate 22 include the surface layer 42 and the surface layer48, respectively, each containing the specified polymer compound havinga silicone chain (a polymer compound that is a polymer containing theconstitutional unit (A) and the constitutional unit (B)), adhesion ofthe migrating particles in the migrating particle group 34 is suppressedby the silicon chain even when the migrating particle group 34 moves andadheres to the facing surfaces. As a result, color reproducibility andhigh contrast are realized.

In the display medium 12 and the display device 10 according to theabove-described exemplary embodiment, the surface electrode 40 and theback electrode 46 are provided on the display substrate 20 and the backsubstrate 22, respectively, so that the particle group 34 is movedbetween the substrates by applying a voltage between the electrodes(i.e., between the substrates) to perform display. However, a displaymode is not limited to this, and, for example, the surface electrode 40may be provided on the display substrate 20, while an electrode may beprovided on the space member so that the particle group 34 is movedbetween the display substrate 20 and the space member by applying avoltage between the electrodes to perform display.

In addition, in the display medium 12 and the display device 10according to the above-described exemplary embodiment, the two types(two colors) of particle groups (34A and 34B) are used as the particlegroup 34. However, one type (one color) of particle group may be used orthree or more types (three or more colors) of particle groups may beused.

EXAMPLES

Although the present invention is described in further detail below withreference to examples, the present invention is not limited to theseexamples.

[Synthesis of Polymer Compound] —Polymer Compound A—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000), 5 parts by massof phenoxyethylene glycol acrylate (NK ester AMP-10G, manufactured byShin-Nakamura Chemical Co., Ltd.), and 90 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound A.

—Polymer Compound B—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000), 10 parts by massof phenoxyethylene glycol acrylate (NK ester AMP-10G, manufactured byShin-Nakamura Chemical Co., Ltd.), and 85 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound B.

—Polymer Compound C—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000), 20 parts by massof phenoxyethylene glycol acrylate (NK ester AMP-10G, manufactured byShin-Nakamura Chemical Co., Ltd.), and 75 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound C.

—Polymer Compound D—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000), 5 parts by massof 2-hydroxy-3-phenoxypropyl acrylate (NK ester 702A, manufactured byShin-Nakamura Chemical Co., Ltd.), and 90 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound D.

—Polymer Compound E—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000), 10 parts by massof 2-hydroxy-3-phenoxypropyl acrylate (NK ester 702A, manufactured byShin-Nakamura Chemical Co., Ltd.), and 85 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound E.

—Polymer Compound F—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000), 20 parts by massof 2-hydroxy-3-phenoxypropyl acrylate (NK ester 702A, manufactured byShin-Nakamura Chemical Co., Ltd.), and 75 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIEN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound F.

—Polymer Compound G—

First, 12 parts by mass of Silaplane FM-0711 (manufactured by ChissoCorporation, weight-average molecular weight Mw=1,000), 28 parts by massof phenoxyethylene glycol acrylate (NK ester AMP-10G, manufactured byShin-Nakamura Chemical Co., Ltd.), and 61 parts by mass of hydroxyethylmethacrylate (manufactured by Wako Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound G.

—Polymer Compound H—

First, 15 parts by mass of Silaplane FM-0711 (manufactured by ChissoCorporation, weight-average molecular weight Mw=1,000), 28 parts by massof phenoxyethylene glycol acrylate (NK ester AMP-10G, manufactured byShin-Nakamura Chemical Co., Ltd.), and 57 parts by mass of methylmethacrylate (manufactured by Wake Pure Chemical Industries, Ltd.) aremixed with 300 parts by mass of isopropyl alcohol (IPA), and 1 part bymass of AIBN (2,2-azobisisobutyronitrile) as a polymerization initiatoris dissolved in the resultant mixture, followed by polymerization at 70°C. for 6 hours under nitrogen. The resultant product is purified byrecrystallization with hexane used as a solvent and then dried toproduce polymer compound H.

—Polymer Compound I—

First, 14 parts by mass of Silaplane FM-0711 (manufactured by ChissoCorporation, weight-average molecular weight Mw=1,000), 26 parts by massof phenoxyethylene glycol acrylate (NK ester AMP-10G, manufactured byShin-Nakamura Chemical Co., Ltd.), and 60 parts by mass of1-vinyl-2-pyrrolidone (manufactured by Wako Pure Chemical Industries,Ltd.) are mixed with 300 parts by mass of isopropyl alcohol (IPA), and 1part by mass of AIBN (2,2-azobisisobutyronitrile) as a polymerizationinitiator is dissolved in the resultant mixture, followed bypolymerization at 70° C. for 6 hours under nitrogen. The resultantproduct is purified by recrystallization with hexane used as a solventand then dried to produce polymer compound I.

—Polymer Compound J (Polymer Compound for Comparative Example)—

First, 5 parts by mass of Silaplane FM-0721 (manufactured by ChissoCorporation, weight-average molecular weight Mw=5,000) and 95 parts bymass of hydroxyethyl methacrylate (manufactured by Wako Pure ChemicalIndustries, Ltd.) are mixed with 300 parts by mass of isopropyl alcohol(IPA), and 1 part by mass of AIBN (2,2-azobisisobutyronitrile) as apolymerization initiator is dissolved in the resultant mixture, followedby polymerization at 70° C. for 6 hours under nitrogen. The resultantproduct is purified by recrystallization with hexane used as a solventand then dried to produce polymer compound J.

[Preparation of Migrating Particle Dispersion Solution] —MagentaParticle (Negatively Charged Particle) Dispersion Solution—

First, 4 parts by mass of silicone-modified polymer KP-545 (manufacturedby Shin-Etsu Chemical Co.) is dissolved in 96 parts by mass of dimethylsilicone oil (KF-96-2CS, manufactured by Shin-Etsu Chemical Co.) toprepare solution X. Next, 10 parts by mass of styrene acryl polymer(neutralized with dimethylethanolamine), 5 parts by mass of an aqueouspigment dispersion solution (Unisuper magenta color, manufactured byCiba Co., pigment content 20% by weight), and 85 parts by mass of purewater are mixed to prepare solution Y. The resultant solutions X and Yare mixed and dispersed/emulsified with an ultrasonic disintegrator(UH-600S manufactured by SMT Co., Ltd.).

Next, the resultant suspension is heated (70° C.) under reduced pressure(2 KPa) to remove moisture and adjust the concentration, therebypreparing a silicone oil dispersion solution (particle solid content 5%by weight) in which migrating particles containing a magenta pigment aredispersed in silicone oil.

As a result of measurement of the volume-average particle diameter(Horiba LA-300: laser light scattering-diffraction particle sizeanalyzer) of the magenta particles (migrating particles) in the preparedmagenta particle dispersion solution, the volume-average particlediameter is 280 nm.

The charge polarity of the magenta particles in the prepared magentaparticle dispersion solution is measured by sealing the dispersionsolution between two electrode substrates and applying a DC current toevaluate the migration direction. As a result, the magenta particles arenegatively charged.

—Cyan Particle (Positively Charged Particle) Dispersion Solution—

First, 95 parts by mass of Silaplane FM-0711 that is a silicone monomer,3 parts by mass of methyl methacrylate, and 2 parts by mass of glycidylmethacrylate are mixed with 50 parts by mass of silicone oil, 0.5 partby mass of azobisvaleronitrile is added as a polymerization initiator tothe resultant mixture, and polymerization is performed to producereactive silicone polymer Z (reactive dispersant) having epoxy groups.The weight-average molecular weight of the polymer is 400,000. Then, a 3mass % silicone oil solution of the reactive silicone polymer Z isprepared. As the silicone oil, dimethyl silicone oil (KF-96L-2CS,manufactured by Shin-Etsu Chemical Co., Ltd.) is used.

Next, a copolymer of N-vinylpyrrolidone and N,N-diethylaminoethylacrylate at a weight ratio of 88/12 is synthesized by usual radicalsolution polymerization and used.

Next, 3 parts by mass of a 10% aqueous solution of the copolymer ismixed with 1 part by mass of an aqueous pigment dispersion solution(Unisuper cyan color, manufactured by Ciba Co., pigment content 26% byweight), and the mixed solution is mixed with 10 parts by mass of the 3mass % silicone solution of the silicone polymer Z. The resultantmixture is stirred for 10 minutes with an ultrasonic disintegrator toprepare a suspension in which an aqueous solution containing the polymerand the pigment is dispersed/emulsified in silicone oil.

Next, the resultant suspension is heated (70° C.) under reduced pressure(2 KPa) to remove moisture, thereby preparing a silicone oil dispersionsolution in which migrating particles containing the polymer and thepigment are dispersed in silicone oil. Further, the dispersion solutionis heated at 100° C. for 3 hours to bond the pigment to the reactivesilicone polymer by reaction.

Next, butyl bromide in an amount corresponding to 50% of the molaramount of N,N-diethylaminoethyl acrylate in the particle solid contentis added to the dispersion solution, and the mixture is heated at 80° C.for 3 hours to quaternize amino groups. Then, purification is performedby repeating settling of the particles using a centrifugal separator andwashing with silicone oil. As a result, a cyan particle dispersionsolution with a particle solid content of 5% by weight is produced.

As a result of measurement of the volume-average particle diameter(Horiba LA-300: laser light scattering-diffraction particle sizeanalyzer) of the cyan particles in the prepared cyan particle dispersionsolution, the volume-average particle diameter is 670 nm.

The charge polarity of the cyan particles in the prepared cyan particledispersion solution is measured by sealing the dispersion solutionbetween two electrode substrates and applying a DC current to evaluatethe migration direction. As a result, the cyan particles are positivelycharged.

[Preparation of Reflecting Particle Dispersion Solution] —White ParticleDispersion Solution—

In a 100-ml three-neck flask provided with a reflux condenser, 5 partsby weight of 2-vinylnaphthalene (manufactured by Nippon Steel ChemicalCo., Ltd.), 5 parts by weight of silicone macromer FM-0721 (manufacturedby Chisso Corporation), 0.3 part by weight of lauroyl peroxide(manufactured by Wako Pure Chemical Industries, Ltd.) serving as aninitiator, and 20 parts by weight of silicone oil KF-96L-1CS(manufactured by Shin-Etsu Chemical Co.) are added, and the resultantmixture is bubbled with nitrogen gas for 15 minutes, followed bypolymerization at 65° C. for 24 hours in a nitrogen atmosphere.

The resultant white particles are adjusted to a solid content of 40 wt %with silicon oil, preparing a white particle dispersion solution. Theparticle diameter of the while particles is 450 nm.

Examples 1 to 9, Comparative Example 1 Formation of Display Medium Cellfor Evaluation

ITO (indium tin oxide) is deposited to a thickness of 50 nm bysputtering to form an electrode on a glass substrate having a thicknessof 0.7 mm, preparing an ITO substrate. Then, a layer of each of thepolymer compounds shown in Table 1 is formed as a surface layer on theITO substrate.

A layer of each of the polymer compounds is formed as follows: First,the polymer compound is dissolved in IPA (isopropyl alcohol) so that asolid content is 4 wt %, and the resultant solution is applied on theITO substrate by a spin coating method and then dried at 130° C. for 1hour to form a polymer compound layer having a thickness of 100 nm.

Two ITO substrates with the surface layers formed as described above areprepared as a display substrate and a back substrate. The displaysubstrate is superposed on the back substrate using a 50 μm Teflon(trade name) sheet as a spacer so that the surface layers face eachother, and the substrates are fixed with a clip.

Then, the magenta particle dispersion solution is injected into a spacebetween the two ITO substrates with the surface layers, thereby forminga display medium cell for evaluation.

—Evaluation—

The prepared display medium cell for evaluation is used, and adirect-current voltage is applied between the substrates (between theITO electrodes) so that the display substrate (the ITO electrodethereof) is positive, and the back substrate (the ITO electrode thereof)is negative. As a result, the negatively charged magenta particlesmigrate to the display substrate side. In this case, the color of themigrating particles which migrate to the display substrate side isobserved from the display substrate side. Then, the optical strength ofa surface of the display substrate is measured while a triangular wave(0.5 V/sec) is applied between the substrates (between the ITOsubstrates thereof) to measure a voltage at a start of color change as athreshold voltage (migration start voltage of migrating particles). Theresults are shown in Table 1.

TABLE 1 Polymer compound Raw material Monomer of Monomer of Monomer ofother constitutional unit (A) constitutional unit (B) constitutionalunit (C) Parts by mass Parts by mass Parts by mass Threshold Type Type(mol %) Type (mol %) Type (mol %) voltage (V) Example 1 A FM0721 5AMP10G  5 HEMA 90 3.8 (0.2 mol %)  (4 mol %) (95.8 mol %) Example 2 BFM0721 5 AMP10G 10 HEMA 85 5.1 (0.2 mol %) (7.5 mol %)  (92.3 mol %)Example 3 C FM0721 5 AMP10G 20 HEMA 75 7.9 (0.2 mol %) (15 mol %) (84.8mol %) Example 4 D FM0721 5 702A  5 HEMA 90 3.5 (0.2 mol %)  (3 mol %)(96.8 mol %) Example 5 E FM0721 5 702A 10 HEMA 85 4.5 (0.2 mol %) (6.5mol %)  (93.3 mol %) Example 6 F FM0721 5 702A 20 HEMA 75 6.8 (0.2 mol%) (14 mol %) (85.8 mol %) Example 7 G FM0711 12  AMP10G 28 HEMA 61 8.0  (2 mol %) (20 mol %)   (78 mol %) Example 8 H FM0711 15  AMP10G 28 MMA57 7.8   (2 mol %) (20 mol %)   (78 mol %) Example 9 I FM0711 14  AMP10G26 VP 60 7.8   (2 mol %) (20 mol %)   (78 mol %) Comparative J FM0721 5No — HEMA 95 1.6 Example 1 (0.7 mol %) (99.3 mol %)

Details in Table 1 are as follows:

FM0721: Silaplane FM-0721 (manufactured by Chisso Corporation,weight-average molecular weight Mw=5,000; structural formula (X1) [n=68,R₁=butyl group], monomer constituting the constitutional unit (A) inwhich Ra₁=methyl group, and n=3)

FM0711: Silaplane FM-0711 (manufactured by Chisso Corporation,weight-average molecular weight Mw=1,000; structural formula (X1) [n=11,R₁=butyl group], monomer constituting the constitutional unit (A) inwhich Ra₁=methyl group, and n=3)

AMP10G: NK ester AMP-10G (manufactured by Shin-Nakamura Chemical Co.,Ltd., phenoxyethylene glycol acrylate)

702A: NK ester 702A (manufactured by Shin-Nakamura Chemical Co., Ltd.,2-hydroxy-3-phenoxypropyl acrylate)

HEMA: 2-hydroxyethyl methacrylate

MMA: methyl methacrylate

VP: 1-vinyl-2-pyrrolidone

The above results indicate that the examples show high thresholdvoltages as compared with the comparative example, and the thresholdvoltages are controlled by adjusting the composition ratio.

Example 10 Formation of Display Medium Cell for Evaluation

ITO (indium tin oxide) is deposited to a thickness of 50 nm bysputtering to form an electrode on a glass substrate having a thicknessof 0.7 mm, preparing an ITO substrate. Then, a layer of each of thepolymer compound C is formed as a surface layer on the ITO substrate.

A layer of the polymer compound C is formed as follows: First, thepolymer compound is dissolved in IPA (isopropyl alcohol) so that a solidcontent is 4 wt %, and the resultant solution is applied on the ITOsubstrate by a spin coating method and then dried at 130° C. for 1 hourto form a polymer compound C layer having a thickness of 100 nm.

Two ITO substrates with the surface layers formed as described above areprepared as a display substrate and a back substrate. The displaysubstrate is superposed on the back substrate using a 50 μm Telfon(trade name) sheet as a spacer so that the surface layers face eachother, and the substrates are fixed with a clip.

Then, a mixture of 10 parts by mass of the white particle dispersionsolution, 5 parts by mass of the cyan particle dispersion solution, and5 parts by mass of the magenta particle dispersion solution is injectedinto a space between the two ITO substrates with the surface layers,thereby preparing a display medium cell for evaluation.

—Evaluation—

The prepared display medium cell for evaluation is used, and a voltageof 20 V is applied for 5 seconds between the substrates (between the ITOelectrodes) so that the display substrate (the ITO electrode thereof) ispositive, and the back substrate (the ITO electrode thereof) isnegative. As a result, the negatively charged magenta particles migrateto the positive-side electrode, i.e., the display substrate side, andthe positively charged cyan particles migrate to the negative-sideelectrode, i.e., the back substrate side. Therefore, magenta color isobserved from the display substrate side.

Then, a voltage of 20 V is applied for 5 seconds between the substrates(between the ITO electrodes) so that the display substrate (the ITOelectrode thereof) is negative, and the back substrate (the ITOelectrode thereof) is positive. As a result, the negatively chargedmagenta particles migrate to the positive-side electrode, i.e., the backsubstrate side, and the positively charged cyan particles migrate to thenegative-side electrode, i.e., the display substrate side. Therefore,cyan color is observed from the display substrate side.

Further, a voltage of 5 V is applied for 5 seconds between thesubstrates (between the ITO electrodes) so that the display substrate(the ITO electrode thereof) is positive, and the back substrate (the ITOelectrode thereof) is negative. As a result, the negatively chargedmagenta particles are held on the positive-side electrode, i.e., thedisplay substrate side, while the positively charged cyan particlesmigrate to the negative-side electrode, i.e., the back substrate side.Therefore, white color is observed from the display substrate side.

Further, under the condition in which the negatively charged magentaparticles migrate to the display substrate side, and the positivelycharged cyan particles migrate to the back substrate side, a voltage of5 V is applied for 5 seconds between the substrates (between the ITOelectrodes) so that the display substrate (the ITO electrode thereof) isnegative. As a result, the negatively charged magenta particles are heldon the display substrate side, while the positively charged cyanparticles migrate to the negative-side electrode, i.e., the displaysubstrate side. Therefore, blue color that is a mixed color of themagenta particles and the cyan particles is observed from the displaysubstrate side.

The above results indicate that in the examples, multi-color display maybe realized by a difference between the threshold voltages of themigrating particles.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

What is claimed is:
 1. A display medium comprising: a pair of substratesat least one of which has transparency and surfaces of which face andare disposed with a space therebetween; a dispersion medium sealedbetween the pair of substrates; a group of migrating particles dispersedin the dispersion medium so as to migrate in the dispersion mediumaccording to an electric field formed between the pair of substrates;and a surface layer provided on at least one of the facing surfaces ofthe pair of substrates and including a polymer compound that is acopolymer containing the following constitutional unit (A) andconstitutional unit (B):

wherein in the constitutional units (A) and (B), X represents a groupcontaining a silicone chain, Ra₁ and Ra₂ each independently represent ahydrogen atom or a methyl group, Rb₂ represents an organic groupcontaining a substituted or unsubstituted phenyl group, n1 and n2 eachrepresent mold of the constitutional unit relative to the wholecopolymer and satisfy 0<n1<50 and 0<n2<80, respectively, and nrepresents a natural number of 1 or more and 3 or less.
 2. The displaymedium according to claim 1, wherein in the constitutional units (A) and(B), Rb₂ represents —Rb₂—O-Ph (wherein Rb₂₁ represents an alkylene grouphaving 1 or more and 11 or less carbon atoms or a hydroxyl-substitutedalkylene group having 1 or more and 11 or less carbon atoms, and Phrepresents a substituted or unsubstituted phenyl group).
 3. The displaymedium according to claim 1, wherein in the constitutional units (A) and(3), n1<n2.
 4. The display medium according to claim 1, wherein thedispersion medium includes an insulating liquid.
 5. The display mediumaccording to claim 1, wherein a volume resistivity of the dispersionmedium is 10¹¹ Ωcm or more.
 6. The display medium according to claim 1,wherein the dispersion medium includes a silicone oil.
 7. The displaymedium according to claim 1, wherein the dispersion medium includes apure water, the volume resistivity of which is 10³ Ωcm or more.
 8. Thedisplay medium according to claim 4, wherein the dispersion mediumincludes a polymer resin.
 9. The display medium according to claim 4,wherein the dispersion medium includes a coloring agent.
 10. The displaymedium according to claim 1, wherein the migrating particles include oneor more of a glass bead, a insulating metal oxide particle, athermoplastic resin particle, a thermosetting resin particle, a resinparticle containing a coloring agent fixed to a surfaces thereof, aparticle of thermoplastic resin containing a coloring agent, a particleof thermosetting resin containing a coloring agent, and a metal colloidparticle having a plasmon coloring function.
 11. The display mediumaccording to claim 1, further comprising a group of reflecting particleswhich have different optical reflection characteristic from themigrating particles and have a property so that the migration particlesmigrate through the reflecting particles between the facing surfaces ofthe pair of substrates when an electric field is applied between thepair of substrates.
 12. A display device comprising: the display mediumaccording to claim 1; and a voltage applying section that applies avoltage between the pair of substrates.